There is a demand for improving the efficiency of a power amplifier in a radio transmitter used for mobile communication base stations in order to miniaturize the apparatus size and reduce the power consumption. Operating the power amplifier with large output generates a distortion due to the nonlinearity of an amplifying device and leaks the power outside a transmission frequency band. Radio wave regulations and wireless standards regulate such leak so as not to interfere with the other radio communications. To reduce nonlinear distortion during a large-output operation, various distortion compensation techniques have been developed as described in non-patent document 1.
One example is the digital predistortion using a digital signal processing device such as an FPGA. FIG. 1 shows its standard construction. A lookup table and a polynomial are used to implement a predistortor 101. The predistortor 101 nonlinearly processes baseband complex input signal Sx=Ix+jQx in a digital region and outputs baseband complex signal Sy=Iy+jQy. A quadrature modulation DA converter (Quadrature DAC) 102 converts the signal into an analog RF signal. A power amplifier (PA) 103 amplifies the signal for output. A quadrature demodulation AD converter (Quadrature ADC)104 converts the output signal into a digital baseband complex signal Sz=Iz+jQz and performs a subtraction with respect to input signal Sx to extract residual distortion signal Se=Sz−Sx. An adaptation algorithm 105 rewrites table values or corrects polynomial coefficients so as to minimize the power for Se. In this manner, nonlinear input/output characteristics of the predistortor 101 are adjusted autonomously.
The performance of the digital predistortion depends on the accuracy of a model provided by the predistortor 101 with reference to nonlinearity of the power amplifier 103. Conventionally, the so-called AM/AM or AM/PM conversion is considered to be a dominant model. The AM/AM or AM/PM conversion model settles an amplitude distortion and a phase distortion depending on an instantaneous value for input amplitude. The model is a function using an amplitude (real number) as input and a complex number as output. Many conventional predistortors are provided with the AM/AM or AM/PM conversion model as represented in patent documents 1 through 3.
Incompleteness of the quadrature modulation DA converter 102 preceding the power amplifier may cause a DC offset, IQ unbalance, or local quadrature error. Such situation reveals a limited capability of the above-mentioned predistortor based on the function model using a real number input and a complex number output. There is a need for introducing a function using a complex number input and a complex number as described in patent document 4 in consideration for an input signal phase.
[Patent document 1] JP-A No. 268150/2001, “Linearizer.”
[Patent document 2] JP-A No. 22659/2000, “OFDM modulator.”
[Patent document 3] JP-A No. 145146/1998, “Nonlinear distortion compensator.”
[Patent document 4] JP-A No. 174332/2003, “Predistortion-type amplifier.”
[Patent document 5] JP-A No. 101908/2005, “Amplifier having predistortion-based distortion compensation function.”
[Non-patent document 1] Nakayama and Takagi. “Power amplifier technique for reduced distortion and improved efficiency.”
[Non-patent document 2] Allen Katz, Marc Franco, “Minimizing Power Amplifier Memory Effects,” Linearizer Technology, Inc., Oct. 7, 2009, pp. 1-30.
[Non-patent document 3] Joel Vuolevi, “Analysis, measurement and cancellation of the bandwidth and amplitude dependence of intermodulation distortion in RF power amplifiers,” Journal of Thesis of Electronics Information and Communications Society, Vol. J-7-C, No. 1 (2004), pp. 49-53.
[Non-patent document 4] Kawaguchi and Akabori. “Distortion compensation using an adaptive predistorter for an amplifier subject to even-ordered distortion,” the Institute of Electronics, Information and Communication Engineers: Article magazine vol. J87-C No. 1, pp. 49-53, April, 2004.
[Non-patent document 5] Muneyasu and Taguchi. “Nonlinear digital signal processing,” Asakura Publishing Co., Ltd., 1999.